A biologically active hydrophobic T-1-conotoxin from the venom of Conus spurius

Edman Degradation Reveals Unequivocal Analysis of the Disulfide Connectivity in Peptides and Proteins

Disulfide bridges in peptides and proteins play an essential role in maintaining their conformation, structural integrity, and consequently function. Despite ongoing efforts, it is still not possible to detect disulfide bonds and the connectivity of multiply bridged peptides directly through a simple and sufficiently validated protein sequencing or peptide mapping method. Partial or complete reduction and chemical cysteine modification are required as initial steps, followed by the application of a proper detection method. Edman degradation (ED) has been used for primary sequence determination but is largely neglected since the establishment of mass spectrometry (MS)-based protein sequencing. Here, we evaluated and thoroughly characterized the phenyl thiohydantoin (PTH) cysteine derivatives PTH-S-methyl cysteine and PTH-S-carbamidomethyl cysteine as bioanalytical standards for cysteine detection and quantification as well as for the elucidation of the disulfide connectivity in peptides by ED. Validation of the established derivatives was performed according to the guidelines of the International Committee of Harmonization on bioanalytical method validation, and their analytical properties were confirmed as reference standards. A series of model peptides was sequenced to test the usability of the PTH-Cys-derivatives as standards, whereas the native disulfide-bonded peptides CCAP-vil, μ-conotoxin KIIIA, and human insulin were used as case studies to determine their disulfide bond connectivity completely independent of MS analysis.

Historical Perspective of the Characterization of Conotoxins Targeting Voltage-Gated Sodium Channels

Marine toxins have potent actions on diverse sodium ion channels regulated by transmembrane voltage (voltage-gated ion channels) or by neurotransmitters (nicotinic acetylcholine receptor channels). Studies of these toxins have focused on varied aspects of venom peptides ranging from evolutionary relationships of predator and prey, biological actions on excitable tissues, potential application as pharmacological intervention in disease therapy, and as part of multiple experimental approaches towards an understanding of the atomistic characterization of ion channel structure. This review examines the historical perspective of the study of conotoxin peptides active on sodium channels gated by transmembrane voltage, which has led to recent advances in ion channel research made possible with the exploitation of the diversity of these marine toxins.

The T-1 conotoxin μ-SrVA from the worm hunting marine snail Conus spurius preferentially blocks the human NaV1.5 channel

Conotoxin sr5a had previously been identified in the vermivorous cone snail Conus spurius. This conotoxin is a highly hydrophobic peptide, with the sequence IINWCCLIFYQCC, which has a cysteine pattern "CC-CC" belonging to the T-1 superfamily. It is well known that this superfamily binds to molecular targets such as calcium channels, G protein-coupled receptors (GPCR), and neuronal nicotinic acetylcholine receptors (nAChR) and exerts an effect mainly in the central nervous system. However, its effects on other molecular targets are not yet defined, suggesting the potential of newly relevant molecular interactions. To find and demonstrate a potential molecular target for conotoxin sr5a electrophysiological assays were performed on three subtypes of voltage-activated sodium channels (Na_V1.5, Na_V1.6, and Na_V1.7) expressed in HEK-293 cells with three different concentrations of sr5a(200, 400, and 600 nM). 200 nM sr5a blocked currents mediated by Na_V1.5 by 33%, Na_V1.6 by 14%, and Na_V1.7 by 7%. The current-voltage (I-V) relationships revealed that conotoxin sr5a exhibits a preferential activity on the Na_V1.5 subtype; the activation of Na_V1.5 conductance was not modified by the blocking effect of sr5a, but sr5a affected the voltage-dependence of inactivation of channels. Since peptide sr5a showed a specific activity for a sodium channel subtype, we can assign a pharmacological family and rename it as conotoxin µ-SrVA.

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

Identification of Novel Conotoxin Precursors from the Cone Snail Conus spurius by High-Throughput RNA Sequencing

Marine gastropods of the genus Conus, comprising more than 800 species, have the characteristic of injecting worms and other prey with venom. These conopeptide toxins, highly diverse in structure and action, are highly potent and specific for their molecular targets (ion channels, receptors, and transporters of the prey's nervous system), and thus are important research tools and source for drug discovery. Next-generation sequencing technologies are speeding up the discovery of novel conopeptides in many of these species, but only limited information is available for Conus spurius, which inhabits sandy mud. To search for new precursor conopeptides, we analyzed the transcriptome of the venous ducts of C. spurius and identified 55 putative conotoxins. Seven were selected for further study and confirmed by Sanger sequencing to belong to the M-superfamily (Sr3.M01 and Sr3.M02), A-superfamily (Sr1.A01 and Sr1.A02), O-superfamily (Sr15.O01), and Con-ikot-ikot (Sr21.CII01 and Sr22.CII02). Six of these have never been reported. To our knowledge, this report is the first to use high-throughput RNA sequencing for the study of the diversity of C. spurius conotoxins.

Structure and allosteric activity of a single-disulfide conopeptide from Conus zonatus at human α3β4 and α7 nicotinic acetylcholine receptors

Conopeptides are neurotoxic peptides in the venom of marine cone snails and have broad therapeutic potential for managing pain and other conditions. Here, we identified the single-disulfide peptides Czon1107 and Cca1669 from the venoms of Conus zonatus and Conus caracteristicus, respectively. We observed that Czon1107 strongly inhibits the human α3β4 (IC₅₀ 15.7 ± 3.0 μm) and α7 (IC₅₀ 77.1 ± 0.05 μm) nicotinic acetylcholine receptor (nAChR) subtypes, but the activity of Cca1669 remains to be identified. Czon1107 acted at a site distinct from the orthosteric receptor site. Solution NMR experiments revealed that Czon1107 exists in equilibrium between conformational states that are the result of a key Ser⁴-Pro⁵ cis-trans isomerization. Moreover, we found that the X-Pro amide bonds in the inter-cysteine loop are rigidly constrained to cis conformations. Structure-activity experiments of Czon1107 and its variants at positions P5 and P7 revealed that the conformation around the X-Pro bonds (cis-trans) plays an important role in receptor subtype selectivity. The cis conformation at the Cys⁶-Pro⁷ peptide bond was essential for α3β4 nAChR subtype allosteric selectivity. In summary, we have identified a unique single-disulfide conopeptide with a noncompetitive, potentially allosteric inhibitory mechanism at the nAChRs. The small size and rigidity of the Czon1107 peptide could provide a scaffold for rational drug design strategies for allosteric nAChR modulation. This new paradigm in the "conotoxinomic" structure-function space provides an impetus to screen venom from other Conus species for similar, short bioactive peptides that allosterically modulate ligand-gated receptor function.

Marine Natural Products from the Yucatan Peninsula

Mexico is one of the three areas of the world with the greatest terrestrial and cultural biological diversity. The diversity of Mexican medicinal flora has been studied for a long time and several bioactive compounds have been isolated. The investigation of marine resources, and particularly the potential of Mexican marine resources, has not been intensively investigated, even though the Yucatan Peninsula occupies 17.4% of the total of the Mexican coast, with great biological diversity in its coasts and the ocean. There are very few studies on the chemistry of natural products from marine organisms that were collected along the coasts of the Yucatan Peninsula and most of them are limited to the evaluation of the biological activity of their organic extracts. The investigations carried out on marine species from the Yucatan Peninsula resulted in the identification of a wide structural variety of natural products that include polyketides, terpenoids, nitrogen compounds, and biopolymers with cytotoxic, antibacterial, antifouling, and neurotoxic activities. This review describes the literature of bioprospecting and the exploration of the natural product diversity of marine organisms from the coasts of the Yucatan Peninsula up to mid-2019.

A sleep-inducing peptide from the venom of the Indian cone snail Conus araneosus

The marine snail Conus araneosus has unusual significance due to its confined distribution to coastal regions of southeast India and Sri Lanka. Due to its relative scarceness, this species has been poorly studied. In this work, we characterized the venom of C. araneosus to identify new venom peptides. We identified 14 novel compounds. We determined amino acid sequences from chemically-modified and unmodified crude venom using liquid chromatography-electrospray ionization mass spectrometry and matrix assisted laser desorption ionization time-of-flight mass spectrometry. Ten sequences showed six Cys residues arranged in a pattern that is most commonly associated with the M-superfamily of conotoxins. Four other sequences had four Cys residues in a pattern that is most commonly associated with the T-superfamily of conotoxins. The post-translationally modified residue (pyroglutamate) was determined at the N-terminus of two sequences, ar3h and ar3i respectively. In addition, two sequences, ar3g and ar3h were C-terminally amidated. At a dose of 2 nmol, peptide ar3j elicited sleep when injected intraperitoneally into mice. To our knowledge, this is the first report of a peptide from a molluscivorous cone snail with sleep-inducing effects in mice. The novel peptides characterized herein extend the repertoire of unique peptides derived from cone snails and may add value to the therapeutic promise of conotoxins.

Conotoxin gene superfamilies

Conotoxins are the peptidic components of the venoms of marine cone snails (genus Conus). They are remarkably diverse in terms of structure and function. Unique potency and selectivity profiles for a range of neuronal targets have made several conotoxins valuable as research tools, drug leads and even therapeutics, and has resulted in a concerted and increasing drive to identify and characterise new conotoxins. Conotoxins are translated from mRNA as peptide precursors, and cDNA sequencing is now the primary method for identification of new conotoxin sequences. As a result, gene superfamily, a classification based on precursor signal peptide identity, has become the most convenient method of conotoxin classification. Here we review each of the described conotoxin gene superfamilies, with a focus on the structural and functional diversity present in each. This review is intended to serve as a practical guide to conotoxin superfamilies and to facilitate interpretation of the increasing number of conotoxin precursor sequences being identified by targeted-cDNA sequencing and more recently high-throughput transcriptome sequencing.

Identification, structural and pharmacological characterization of τ-CnVA, a conopeptide that selectively interacts with somatostatin sst3 receptor

Conopeptides are a diverse array of small linear and reticulated peptides that interact with high potency and selectivity with a large diversity of receptors and ion channels. They are used by cone snails for prey capture or defense. Recent advances in venom gland transcriptomic and venom peptidomic/proteomic technologies combined with bioactivity screening approaches lead to the identification of new toxins with original pharmacological profiles. Here, from transcriptomic/proteomic analyses of the Conus consors cone snail, we identified a new conopeptide called τ-CnVA, which displays the typical cysteine framework V of the T1-conotoxin superfamily. This peptide was chemically synthesized and its three-dimensional structure was solved by NMR analysis and compared to that of TxVA belonging to the same family, revealing very few common structural features apart a common orientation of the intercysteine loop. Because of the lack of a clear biological function associated with the T-conotoxin family, τ-CnVA was screened against more than fifty different ion channels and receptors, highlighting its capacity to interact selectively with the somatostatine sst3 receptor. Pharmacological and functional studies show that τ-CnVA displays a micromolar (Ki of 1.5μM) antagonist property for the sst3 receptor, being currently the only known toxin to interact with this GPCR subfamily.

Identification, by RT-PCR, of four novel T-1-superfamily conotoxins from the vermivorous snail Conus spurius from the Gulf of Mexico

cDNA was prepared from the venom duct of a single Conus spurius specimen collected near the coast of Campeche, Mexico. From it, PCR products were generated, sequenced, and predicted to encode eight distinct precursors of T-1-conotoxins. These precursors contain five different mature toxins, of which four are novel and one (sr5a) has been previously purified and characterized from the venom of this species. Three of the novel toxins are very similar to sr5a: two have one amino acid substitution at position 8, whereas the other is predicted to have one additional residue at the C-terminus; the fourth toxin has five amino acid substitutions and is predicted to have two additional residues at the C-terminus. In general, the precursors include a 22-residue signal peptide, a 24-residue "pro" region, and a 13- to 16-residue mature toxin region; however, the C-termini of two mature toxin regions are predicted to be altered by post-translational processing. Three precursors lack, in the same positions, 15 amino acid residues included in the "pre" (one residue) and "pro" (14 residues) regions, which suggests the existence of an exon encoding the last signal peptide residue and the first 14 residues of the "pro" region. Phylogenetic analysis indicates that the T-1-conotoxin precursors and mature toxins of C. spurius are more similar to certain precursors and toxins from molluscivorous Conus species than to any precursors and toxins from vermivorous cones. The results reported here will be useful for synthesizing the novel toxins in order to identify their molecular targets.

mr1e, a conotoxin from Conus marmoreus with a novel disulfide pattern

Conotoxins are well known for their highly variable structures and functions. Here we report the identification of a novel conotoxin named mr1e from Conus marmoreus. mr1e is composed of 11 amino acid residues cross-linked by two disulfide bonds (CCHSSWCKHLC). The spacing of intercysteine loops in mr1e is exactly the same as that in alpha4/3 conotoxins. However, the native mr1e peptide co-eluted on reverse-phase HPLC with the regioselectively synthesized ribbon disulfide linkage isomer (C1-C4, C2-C3) but not the globular linkage isomer (C1-C3, C2-C4). Although this peptide has the same disulfide connectivity as the chi-conotoxins, their sequences do not share significant homology. Thus, mr1e could be defined as a novel conotoxin family. By intracranial injection into mice, mr1e showed an excitatory effect. The characterization of mr1e certainly enriches our understanding of conotoxins, and also opens an avenue for further structural and functional investigation.

Conus ventricosus venom peptides profiling by HPLC-MS: a new insight in the intraspecific variation

Conus is a genus of predatory marine gastropods that poison the prey with a complex mixture of compounds active on muscle and nerve cells. An individual cone snail's venom contains a mixture of pharmacological agents, mostly short, structurally constrained peptides. This study is focused on the composition of the venom employed by Conus ventricosus Gmelin, 1791, a worm-hunting cone snail living in the Mediterranean Sea. For this purpose, LC coupled to MS techniques has been successfully used to establish qualitative and quantitative differences in conopeptides from minute amounts of venom ducts. We were able to prove variability in the venom conopeptide complement, possibly related to different trophic habits of the species in the Mediterranean Sea. Moreover, the information-rich MS techniques enabled us to identify two novel C. ventricosus peptides, here named Conotoxin-Vn and -Conotoxin-Vn. On the basis of the structural data collected so far, we suggest that Conotoxin-Vn is a conopeptide belonging to the -family that recognizes calcium channels through a specific pharmacophore. Similarly, molecular modeling data suggest that -Conotoxin-Vn should represent a competitive antagonist of neuronal nicotinic acetylcholine receptors (nAChRs).

Isolation and characterization of a T-superfamily conotoxin from Conus litteratus with targeting tetrodotoxin-sensitive sodium channels

A T-1-conotoxin, lt5d, was purified and characterized from the venom of vermivorous hunting cone snails Conus litteratus. The complete amino acid sequence of lt5d (DCCPAKLLCCNP) has been determined by Edman degradation. With two disulfide bonds, the calculated average mass is 1274.57 Da, which is confirmed by MALDI-TOF mass spectrometry (average mass 1274.8778). Under whole cell patch-clamp mode, lt5d inhibits tetrodotoxin-sensitive sodium currents on adult rat dorsal root ganglion neurons, but has no effects on tetrodotoxin-resistant sodium currents. The inhibition of TTX-sensitive sodium currents by lt5d was found to be concentration-dependent with the IC(50) value of 156.16 nM. Thus, this is the first T-superfamily conotoxin identified to block TTX-sensitive sodium channels.

Identification of six novel T-1 conotoxins from Conus pulicarius by molecular cloning

Cone snails are a group of ancient marine gastropods with highly sophisticated defense and prey strategies using conotoxins in their venom. Conotoxins are a diverse array of small peptides, mostly with multiple disulfide bridges. Using a 3' RACE approach, we identified six novel peptides from the venom ducts of a worm-hunting cone snail Conus pulicarius. These peptides are named Pu5.1-Pu5.6 as their primary structures show the typical pattern of T-1 conotoxin family, a large and diverse group of peptides widely distributed in venom ducts of all major feeding types of Conus. Except for the conserved signal peptide sequences in the precursors and unique arrangement of Cys residues (CC-CC) in mature domains, the six novel T-1 conotoxins show remarkable sequence diversity in their pro and mature regions and are, thus, likely to be functionally diversified. Here, we present a simple and fast strategy of gaining novel disulfide-rich conotoxins via molecular cloning and our detailed sequence analysis will pave the way for the future functional characterization of toxin-receptor interaction.

Sequencing of T-superfamily conotoxins from Conus virgo: pyroglutamic acid identification and disulfide arrangement by MALDI mass spectrometry

De novo mass spectrometric sequencing of two Conus peptides, Vi1359 and Vi1361, from the vermivorous cone snail Conus virgo, found off the southern Indian coast, is presented. The peptides, whose masses differ only by 2 Da, possess two disulfide bonds and an amidated C-terminus. Simple chemical modifications and enzymatic cleavage coupled with matrix assisted laser desorption ionization (MALDI) mass spectrometric analysis aided in establishing the sequences of Vi1359, ZCCITIPECCRI-NH(2), and Vi1361, ZCCPTMPECCRI-NH(2), which differ only at residues 4 and 6 (Z = pyroglutamic acid). The presence of the pyroglutamyl residue at the N-terminus was unambiguously identified by chemical hydrolysis of the cyclic amide, followed by esterification. The presence of Ile residues in both the peptides was confirmed from high-energy collision induced dissociation (CID) studies, using the observation of w(n)- and d(n)-ions as a diagnostic. Differential cysteine labeling, in conjunction with MALDI-MS/MS, permitted establishment of disulfide connectivity in both peptides as Cys2-Cys9 and Cys3-Cys10. The cysteine pattern clearly reveals that the peptides belong to the class of T-superfamily conotoxins, in particular the T-1 superfamily.

De novo sequencing and disulfide mapping of a bromotryptophan-containing conotoxin by Fourier transform ion cyclotron resonance mass spectrometry

T-1-family conotoxins belong to the T-superfamily and are composed of 10-17 amino acids. They share a common cysteine framework and disulfide connectivity and exhibit unusual posttranslational modifications, such as tryptophan bromination, glutamic acid carboxylation, and threonine glycosylation. We have isolated and characterized a novel peptide, Mo1274, containing 11 amino acids, that shows the same cysteine pattern, -CC-CC, and disulfide linkage as those of the T-1-family members. The complete sequence, GNWCCSARVCC, in which W denotes bromotryptophan, was derived from MS-based de novo sequencing. The FT-ICR MS/MS techniques of electron capture dissociation (ECD), infrared multiphoton dissociation, and collision-induced dissociation served to detect and localize the tryptophan bromination. The bromine contributes a distinctive isotopic distribution in all fragments that contain bromotryptophan. ECD fragmentation results in the loss of bromine and return to the normal isotopic distribution. Disulfide connectivity of Mo1274, between cysteine pairs 1-3 and 2-4, was determined by mass spectrometry in combination with chemical derivatization employing tris(2-carboxyethyl)phosphine, followed by differential alkylation with N-ethylmaleimide and iodoacetamide. The ECD spectra of the native and partially modified peptide reveal a loss of bromine in a process that requires the presence of a disulfide bond.